Recently, with respect to powertrain control for a vehicle, functions for performing complex control are expected to be sophisticated in order to achieve both a comfortable drive and low-fuel consumption. Furthermore, as the number of electronic control devices is increased, the number of wiring harnesses may be increased and securing a space for mounting various devices becomes difficult. Thus, a module including a mechanical functional component as an object to be controlled integrated with an electronic control device is expected.
Manufacturing a module including a mechanical functional component integrated with an electronic control device has been studied. For example, an electronic control device is mounted in a valve body constructing an oil-hydraulic circuit inside a transmission integrally with a solenoid and various sensors. As examples of such an electronic control device, each of JP-A-2004-119465 corresponding to US 2007/148341 and JP-A-2008-084978 corresponding to US 2008/074829 describes an electronic control device in which a circuit board is sealed by a molding resin.
In an electronic control device described in JP-A-2004-119465, a circuit board on which electronic components are mounted is bonded to a base member constructed of a laminated body of iron-nickel alloy sandwiched by copper, and is cast by a molding resin other than a part of an external connecting terminal and a flange portion arranged at the base member.
In an electronic control device described in JP-A-2008-084978, a circuit board having electronic components on both surfaces thereof is bonded to a base member made of a sintered compact of mixed powder of aluminum and silicon carbide, and is cast by a molding resin other than a part of an external connecting terminal and a part of the base member. The electronic control device has a half mold structure, that is, a surface of the base member that is opposite to a surface of the base member to which the circuit board is bonded is exposed to outside. Thus, radiation performance of the electronic components is improved.
As described in JP-A-2004-119465 and JP-A-2008-084978, a technique to mold an electronic circuit assembly is a conventional technique known for an IC package. However, in a large-size component such as a control circuit for a transmission, stress due to the difference between the amount of heat contraction of a molding resin and components such as a circuit board and a base member is so large that peeling at an interface may be generated. The stress is generated from an outer edge portion of a molded object toward the center thereof.
JP-A-2004-119465 describes a full mold structure, that is, the molding resin covers also a rear surface of the base member. A warpage due to a difference between a linear expansion coefficient of the base member and that of the molding resin is prevented by the full mold structure. Further, the base member is constructed of the laminated body of iron-nickel alloy sandwiched by copper. Therefore, the linear expansion coefficient of the base member is set to be approximately equal to that of the molding resin, and peeling stress is decreased.
JP-A-2008-084978 describes that a size of the circuit board is reduced as much as possible by arranging the electronic components on both surfaces of the circuit board so as to reduce the stress. Further, the base member is made of the sintered compact of mixed powder of aluminum and silicon carbide. Therefore, a linear expansion coefficient of the base member is set to be approximately equal to that of the molding resin, and an elastic modulus can be lowered.
In JP-A-2004-119465 and JP-A-2008-084978, peeling between the circuit board, the base member and the molding resin is prevented by modifying the material or the structure. In contrast, with respect to a large-size molded device, JP-A-2006-041071 describes that a circuit board is covered by an adhesion improving member such as polyamide-imide generally. That is, the circuit board is covered by the adhesion improving member, which has a lower elastic modulus than the molding resin and adhesion. Therefore, sharing stress at an interface of the molding resin is decreased, and the molding resin is unlikely to be peeled off the circuit board.
However, in the full mold structure described in JP-A-2004-119465, the molding resin is arranged on an opposite surface of a surface to which the circuit board is bonded in the base member. Thus, heat generated at the electronic components is released to outside through the flange portion arranged at the base member and exposed to outside from the molding resin, so that radiation performance may be decreased.
Further, in the half mold structure described in JP-A-2008-084978, because a part of the base member is exposed from the molding resin, the radiation performance is improved. However, one surface of the base member is covered by the molding resin, and another surface thereof is not covered by the molding resin. That is, even if the molding resin contracts at one surface of the base member, because the molding resin is not arranged on another surface thereof, it becomes difficult to maintain balance of contraction in the molded device. Thus, it is difficult to prevent the warpage of the molded device as a whole, and reliability in adhesion at an interface of the molding resin may be decreased.
In both structures described in JP-A-2004-119465 and JP-A-2008-084978, the linear expansion coefficients of the molding resin and the base member can be set to be about 8 to 10 ppm/° C. However, a linear expansion coefficient of ceramic used for the circuit board is about 5 to 7 ppm/° C., and it is difficult for the linear expansion coefficients of the molding resin and the base member to set to be approximately equal to that of the circuit board.
Further, a material for a ceramic board is calcined and divided at a V-groove so that the ceramic board as the circuit board is obtained. Thus, an edge portion of the ceramic board takes the form of an acute angle, and the stress is likely to be concentrated at the edge portion. Further, an elastic modulus of the ceramic board is about 250 GPa, and the great stress may be generated at the edge portion.
In the case where the adhesion improving member is used as described in JP-A-2006-041071, the adhesion improving member forms a fillet and is filled in a concavo-convex portion around components on the circuit board by a capillary phenomenon, and thereby the adhesion improving member can be thickened. Thus, it is considered that the stress can be reduced due to low elasticity. However, because a viscosity of the adhesion improving member is low such that the circuit board is uniformly coated with the adhesion improving member by spraying, a thickness of the adhesion improving member at a flat portion such as the edge portion may become small. Although adhesion between the molding resin and the circuit board can be improved, there is a limit to reduce the stress due to the low elasticity.
The peeling stress in the molding resin is generated from an outer edge portion of the circuit board toward the center thereof, and increases with increasing distance from the center of the circuit board. Therefore, as described above, in the case where a circuit size is enlarged by unifying various components and the circuit board becomes larger, the peeling stress applied to the outer edge portion of the circuit board may become larger. Therefore, even when the base member is made of the same material with the molding resin and the adhesion between the molding resin and the circuit board is improved, interfacial peeling at the edge portion of the circuit board cannot be prevented.
If the molding resin is peeled off the circuit board, a position of the molding resin is displaced with respect to the circuit board. Thus, a bonding wire of the electronic component mounted on the circuit board may be cut, for example, so that electronic failure may generate.